Nowadays, the plastics are the most commonly used materials and the main reason is their versatility for many applications, which have allowed replacing, partly or completely, materials such as steel, glass, wood, aluminum, among others. According to the study “Analysis of production, demand and recovery of plastics” by the Association of Plastics Manufacturers (EuPR), from 1950 to 2010, in the world, the average annual increment in the production of plastics materials has been of 9%, as a consequence of a continuous innovation. According to Subramanian (Subramanian P. M. (2000). Plastics recycling and waste management in the US. Resources, Conservation and Recycling 28: 253-263), the polymers with higher consumption are: Low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP) and poly (ethylene terephthalate) (PET).
The blend of wasted polymers such as PET and polyolefins (polyethylene and polypropylene), may provide an alternative route for the production of recycled materials with satisfactory costs, good yields and wider application potential. Nevertheless, one of the drawbacks for blends of PET and polyethylene is that they are thermodynamically immiscible, so the blends exhibit a coarse morphology giving very poor mechanical properties. Furthermore, due to the chemical structure of polyethylene (PE), which lacks of functional groups, it is difficult to interact with most of polymeric materials. Good blends with these polymers can be achieved by appropriate methods of compatibilization and processing technologies by improving interfacial adhesion and dispersion in blends. Thus, a composite material with better thermal and mechanical properties compared to the starting polymers can be obtained.
The compatibility of immiscible blends can be improved by one of the following ways:
The addition of a third component having a segment that is able to have a specific interaction and/or chemical reaction with the components of the blend (for example, block or graft copolymers).
In the case of PET, mixing it with suitable functionalized polymers capable of carrying out a chemical reaction with the functional group of the polyester.
Torres disclosed in 2001 (Torres N, Robin J J, Boutevin B (2001). Chemical modification of virgin and recycled poly(ethylene terephthalate) by adding of chain extenders during processing. J App Polym Sci 81: 2377-2386), that synthesis of functionalized polyolefins GMA by copolymerization in solution is relatively expensive to be used for large amounts in products such as blends of HDPE/PET. Studies by Champagne (1999) (Champagne M F, Huneault M A, Row C, Peyrel W (1999). Reactive compatibilization of polypropylene/polyethylene terephthalate blends. Polym Eng Sci. 39: 976-984), reported that the use of maleic resins do not provide good adhesion between the PE and PET. However, other investigations have shown better adhesion between the two phases of immiscible blends of PET/PE giving an increase with respect to the reference blend by using maleic anhydride grafted polyethylene.
Currently, there are different types of commercial compatibilizers like random polymers, block copolymers, grafted polymers and functionalized polymers. The most used compatibilizers are polyolefins grafted with maleic anhydride (MA) like PE-g-MA. Other commonly used compatibilizers are block polymers include styrene-ethylene/butylene-styrene (SEBS), ethylene vinyl acetate (EVA), to compatibilize blends of polyethylene/polycarbonate.
Nowadays, the majority of the research done on the compatibility of immiscible polymer blends often use compatibilizers based on graft or copolymers type, which usually have a limited number of available functional groups to interact with the polymers of the blend.
The field of the interpenetrated polymer networks (IPN) gained interest since the 70's decade, when they were described for the first time. The most common IPN structures are mainly based on polyurethane (PU), polysiloxanes, polyesters (PEST), epoxy resins, and polyacrylates (PAcr) (Utracki, L. A. Polymer Blends Handbook. Dordrecht: Kluwer Academic Publishers, 2002. Chapter 6) In order to have an ideal IPN structure, the components are partially bonded at a molecular scale, so they cannot be split unless the chemical bonds are broken. The crosslinking of every IPN is only controlled by the type and concentration of the employed crosslinking agent. In the case of real IPN structures, it's very unlikely that the A-polymer crosslinks or grafts the B-polymer, due to the preparation method which involves 2 stages (by a free radical polymerization). Crosslinking agents usually have two or more double bonds, so during the second stage of polymerization this double bonds can act as grafting sites (so is the case of the polybutadiene), forming sites in which B-polymer can easily graft; furthermore, if A-polymer has an α-hydrogen (such as polybutylacrylate), it can be displaced to form additional bonds. Nowadays, in many cases (but not in the case of the present invention), a graft site is created in purpose, to improve the blending of B-polymer in A-polymer; this technique is known as reactive compatibilization.
For thermoplastics IPN's, there is physical crosslinking, more than chemical bonding; frequently, the physical crosslinking is based on triblock copolymers, being the TPE the most common choice in addition to ionomers or semi-crystalline materials. As examples of this group there are some relevant blends such as ethylene-propylene-diene monomer/polypropylene (EPDM/PP), nitrile butadiene rubber/polyamide (NBR/PA), polyurethane/polyamide (PU/PA), styrene-ethylene-butylene-styrene/polyamide (SEBS/PA), ethylene-propylene-diene/polybuthyleneterephtalate (EPDM/PBT), and epichlorohydrin/polyamide (ECH/PA). Utracki, L. A. Polymer Blends Handbook. Dordrecht: Kluwer Academic Publishers, 2002. Chapter 5).
Nishi and Kotaka ((1985) Complex-Forming Poly(oxyethylene):Poly(acrylic acid) Interpenetrating Polymer Networks. Preparation, Structure, and Viscoelastic Properties. Macromolecules, Vol. 18, No. 8, 1985) show that for improving the compatibility of given polymer pairs, various attempts have been made. One such attempt was the introduction of cross-links within each component to prepare interpenetrating polymer networks (IPNs) or semi-interpenetrating polymer networks (SIPNs).
According to the IUPAC, the Interpenetrating polymer network is an intimate combination of two polymers both in network form, at least one of which is synthesized and/or cross-linked in the immediate presence of the other. And the Semi-interpenetrating polymer network is a Combination of two polymers, one cross-linked and one linear, at least one of which was synthesized and/or cross-linked in the immediate presence of the other.
Other TPE with interesting properties, but it has not been widely used is the styrene-ethylene-propylene-styrene triblock copolymer (SEPS). The SEPS has been used as compatibilizer of polymer blends like Polypropylene/Polystyrene (PP/PS); Polypropylene/High Density Polyethylene (PP/HDPE); and Polypropylene/Polycarbonate/Ethylene-octene copolymer (PP/PC/POE) (Sung-Goo Lee, Jae Heung Lee, Kil-Yeong Choi, John Moon Rhee. 1998. Glass transition behavior of polypropylene/polystyrene/styrene-ethylene-propylene block copolymer blends. Polymer Bulletin 40, 765-771), (Dhibar, A. K. Kim J. K., Khatua B. B. 2011. Cocontinuous Phase Morphology of Asymmetric Compositions of Polypropylene/High-density Polyethylene Blend by the Addition of Clay. Journal of Applied Polymer Science, Vol. 119, 3080-3092) (Dai S., Ye L. 2008. Effect of SEPS as a Novel Compatibilizer on the Properties and Morphology of PP/PC/POE Blends. Journal of Applied Polymer Science, Vol. 108, 3531-3541).
In all these studies, it is important to mention that they consider compatibilizer quantity between 1 to 10% by weight. In addition, in some cases, the SEPS has been used to improve toughness of polymeric materials like PP (Matsuda Y., Hara M., Mano T., Okamoto K. and Ishikawa M. (2005). The Effect of the Volume Fraction of Dispersed Phase on Toughness of Injection Molded Polypropylene Blended With SEBS, SEPS, and SEP. POLYMER ENGINEERING AND SCIENCE, DOI 10.1002/pen.20298)
In the case of the Poly (acrylic acid) (PAA), (Sun J. Y., Hu G. H. and Lambla M. Kotlar H. K. (1996) In situ compatibilization of polypropylene and poly(butylene terephthalate) polymer blends by one-step reactive extrusion. Polymer Vol. 37 No. 18, pp. 4119-4127), showed that the polypropylene/poly(butylene terephthalate) (PP/PBT) blends have been compatibilized by using a one-step reactive extrusion process with the addition of monomer of acrylic acid, which are potentially reactive towards the carboxylic and/or hydroxyl groups at the chain ends of the PBT.
The purpose of compatibilizing a polymer blend is to reduce the interfacial tension between the two phases (A-polymer and B-polymer interface), in order to ease the dispersion, and stabilize the morphology generated when blending and during the following process steps. It's used also to improve the adhesion between both polymers, improving the stress transfer, enhancing the mechanical properties of the final product. The efficiency of the added compatibilizer is determined by its preferential localization in the interphase. The compatibilizer must be designed depending of the polymer blend nature. There are several compatibilization strategies for polymer blends, for example, the most common of them is by means of the addition of a third component which is miscible in both structures (co-solvent). In the case of polyester/polyolefins blends (so is the case of the present invention), and more particularly the case of PET/polyolefins blends, the common solution is to graft polyolefins with maleic anhydride (MA). So, in a general way, there are three compatibilizing methods which may be described as follows:
By the simple addition of a compatibilizing agent (like the Phenoxy™ added for PBT/PMMA blends). It results very useful to identify co-polymers that have certain morphologies capable of establish a co-continuity within the phases of the compatibilized blend, for example the SEBS added to a PP/Polycarbonate (PP/PC) blend.
Reactive compatibilization; in this case, block or graft co-polymers are generated, so they can form chemical bonds through the interphase. Nowadays, this is the most popular method, present in about the 90% of the commercial blends.
Physical compatibilization, in which a fine morphology is generated, and stabilized by nucleation crystallization.
In polymer recycling field there are several “universal compatibilizers” for waste polymers, which are typically multi-component polymers with moieties soluble in some components of the polymeric blend, or in certain cases, the moieties are able to form chemical bonds with them. Due to this universality of these compounds, it is really expensive to employ them. The most common substances employed to modify some polymers are also good compatibilizers which offer a cheaper option for polymer blends. These materials are formulated for specific polymer mixtures, for example Blendex™ (an acrylic based additive, for PEST) which works as a compatibilizer from the polybutadiene family for styrenic resins, PVS, TPU, and PET; or FUSABOND™, a polyolefin modified with MA, employed for PET/polyolefins blends; and finally, Vector™, which is a block copolymer with stabilizers, designed for polyolefins/PS blends. (Utracki, L. A. Polymer Blends Handbook. Dordrecht: Kluwer Academic Publishers, 2002. Chapter 16)
Document GB1581167A discloses the synthesis of a composite from thermoplastic resins (PBT and/or polyolefins), blended with a block co-polymer (SEBS for this case), which have an IPN structure. Document CA1111987A1 refers to a polymeric blend comprising a partially hydrogenated block co-polymer with a thermoplastic resin. The partially hydrogenated block copolymer is limited to have two mono alkenyl arene terminal groups, and the intermediate block polymer is a conjugated diene (like S—B—S). The ratio of this composite is: 5-95% wt of the partially hydrogenated block co-polymer, and 95-5% wt of a thermoplastic resin whose melting temperature range is 120-300° C. The particular composite of this example comprises a lineal polyolefin (poly methyl pentene, PMP), and a polyester (PET or PBT), compatibilized by a 40-70% wt of the partially hydrogenated block co-polymer. This composite, in contrast to the present invention, does not have poly acrylic polymer improving the compatibilizer, which forms the IPN structure with the partially block-copolymer of the present invention, in order to achieve the polymer blend.
WO0327366A1 discloses a polymeric fiber which comprises a TPE; the structure comprises a nucleus (which may be any of the following: polyetheramides, polyurethanes, elastomeric polyolefins, polyester, or styrenic TPE), and a disperse phase which is a non-elastomeric thermoplastic polymer (like non-elastomeric polyolefins, polyesters, or polyamides); the ratio of such blends is 10-70% wt of the disperse phase, and 90-30% of the nucleus. The difference between this and the present invention is that, even though the non-elastomeric polymer may be polyethylene or a polyester (such as PET), and that the nucleus may be an styrenic TPE, there is no mention neither about the IPN's structures, nor about poly acrylic acid (PAcr) within the composite.
WO2013075241A1 describes a polyethylene/polypropylene blend (PE/PP), ratio 0.5-0.5% wt, where the polymers form a “droplet-in-droplet” microstructure, best described as one polymer is encapsulated into another polymeric layer, forming layers just like “onion skin-like” structure; this structure is achievable by means of an interfacial agent, which in this case is an EPDM, or any ethylene-propylene co-polymer.
CA1334876C describes that a styrenic co-polymer can modify the mechanical properties of a polymeric blend. Said co-polymer is ethylene-propylene-diene grafted with glycidyl acrylate or glycidyl methacrylate, and the nucleus elastomer has a poly alkyl acrylate linked to the polystyrene structure, all these conforming an IPN structure; the difference between this example and the present invention lies on the fact that in the instant case the polymers comprising the IPN are not grafted, besides the acrylic polymer and the TPE do not have chemical bonds between them, which makes the structure less complex, even to synthetize.
Document CA2029032A1 presents a composite which comprises a thermoplastic resin, forming an IPN structure, whose matrix is a thermoplastic resin; the manufacturing method for this composite is by means of the “melt-blending” technique. The composite comprises: A) polyolefins (PE, PP), aromatic polyesters (PET, PBT), polyacetal, polystyrene, or polyester; B) organic or inorganic fibers, like polystyrene resins; and C: a thermoplastic additive that plays the role of an interfacial agent between A and B phases. As shown in the description, the interfacial agent does not have an acrylic component which can form the IPN structure for the sole compatibilizer. In addition, the continuous phase in the present invention is composed by polyacrylic polymer.
WO9417226A1 shows a polymer blend which main phase comprises polypropylene (or any polyester, polyacrylate, polyolefin, or polyamide), and the disperse phase comprises polyethylene. PE/PP is one of the most common polymer blends attempts. In this case, the composition may include up to 20% wt of any adhesive which promotes the adhesion, such as poly ethylene vinyl acetate, ethyl methacrylate, ethylene ethyl acrylates; even though the composition includes acrylates as compatibilizer-like additive, it is not suggested or disclosed how it would be improved when the additive is a mixture of acrylic and a TPE with an IPN structure, such as in the case of the present invention.
Finally, document WO9425526A1 discloses the most traditional method (even nowadays) to create polymer blends (as shown in the previously cited documents). The process consists on modifying or functionalizing polymers, by grafting them with some other substances (such as maleic anhydride, MA), to be able to blend them; contrary to the present invention, where neither the acrylic polymer, nor the TPE, were grafted, making the manufacturing process less complex, due to the fact that neither the compatibilizer, nor the non-compatible polymers require a previous treatment (such as grafting).
In light of the above, the present invention discloses a new compatibilizer based on an interpenetrating polymer network (IPN) because it is a combination of two or more polymers in network form, where at least one of them is synthesized and/or cross-linked in the immediate presence of the other. This type of compatibilizer has a higher number of functional groups and also will contain chemical structure that will interact with the blend components. Also, the method for blending recyclable plastics is new, since the novel compatibilizer is needed without additional pre-treatment processes (such as grafting).